I'd be grateful for any suggestions as to why one key - and only one key -
on my PC keyboard should suddenly have stopped working.

The key in question is f11 which I'm trying to program with a Word macro.
Until yesterday it was working perfectly with a slightly different version
of the same macro.

The keyboard is an admittedly quite elderly (and expensive, when bought)
Cherry model which in every other respect is working fine.

I've tried opening the case and cleaning the keyboard's innards, both with a
brush and an air spray. There's no visible damage either to the underside
of the board or (with the key cap removed) to the individual switch.

Thanks.